According to include/linux/compiler-gcc4.h I should use a gcc version higher
than 4.1.0

I used gcc 4.1.2 instead of 4.1.0 in the crosstool script and built. Then I
rebuild the kernel. It worked fine.
I also tried to build 2.6.18.8 with 4.1.0 and it worked. Somewhere between
2.6.15.4 to 2.6.30.1 the minimum minor version of
gcc was changed from 1 to 2.